The disclosed embodiments of the present invention relate to wireless signal transmission, and more particularly, to a transmitter system with a digital phase rotator used for applying a digital phase rotation to a constellation data and a related signal transmission method thereof.
The peak-to-average power ratio (PAPR) of a transmit signal is an important parameter in a communication system because it determines the back-off factor that needs to be applied to the power amplifier in order to avoid clipping of the transmit signal and hence spectral regrowth. In general, the power amplifier is used to boost radiation power out of an antenna. The transfer function of the power amplifier should be linear to avoid undesired distortion. In practice, the power amplifier may exhibit a linear transfer characteristic only at a low input signal level. When the input signal level is too high, nonlinear distortion occurs, resulting in clipping and spectral regrowth. Therefore, to maintain tight spectral confinement of the transmit signal, the efficiency of the power amplifier has to be reduced. In other words, when the PAPR is high, this puts a stringent requirement on the power amplifier, and reduces the efficiency in the sense that larger back-off is needed to maintain the linearity.
Because of the advance of the deep sub-micro complementary metal-oxide-semiconductor (CMOS) process, digital circuits become smaller and consume less power. That is, the process scaling allows more transistors implemented in the same area or a smaller area needed for implementing the same number of transistors. Hence, to reduce the device size and prolong battery life for the portable device such as a mobile phone, it is desirable to implement circuits in deep sub-micron CMOS process. Regarding the radio design, there is a strong push to convert the radio-frequency (RF)/analog intensive radio parts into more digital intensive designs to take full advantage of the process scaling. For example, a digital radio-frequency (RF) transmitter may be employed in a communication system. However, the transmission PAPR would reduce the allowable output power range of the digital RF transmitter. Thus, there is a need for an innovative design which is capable of controlling and minimizing the digital RF transmitter's PAPR to achieve better power efficiency.